Method for preparing nanoparticles comprising cerium oxide and zirconium

ABSTRACT

This invention provides a method for preparing nanoparticles comprising cerium oxide and zirconium and having a narrow size distribution. The method comprises providing a first aqueous solution comprising zirconium oxychloride and providing a second aqueous solution comprising a first component which is either cerium nitrate or hexamethylenetetramine. The second aqueous solution is added to the first aqueous solution to form a first mixture. A third aqueous solution comprising a second component which is either cerium nitrate or hexamethylenetetramine, and which is different from the first component, is added to the first mixture to form a second mixture. The second mixture is maintained at a temperature no higher than about 320° K to form nanoparticles. The nanoparticles are then separated from the second mixture and sintered in air at a temperature ranging between about 500° and about 1100° C. The nanoparticles obtained by the method of the invention are at least in part crystalline.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The invention is directed to a method for the preparation ofnanoparticles comprising cerium oxide and zirconium. In particular, theinvention is directed to a method for the preparation of nanoparticlescomprising cerium oxide and zirconium having a narrow size distribution.

2. Background Information

Cerium oxide in the form of fine particles is useful as a catalyst forpolymerization, for reforming fuels, and for abating polluting gas inautomobile exhaust system. The catalyst acts as an oxygen pressureregulator in the reduction of NO_(x) to molecular nitrogen, theoxidation of hydrocarbons and carbon monoxide to water and carbondioxide, and the conversion of H₂S to H₂ and S.

Cerium oxide has been used as a catalyst-component for the recombinationof hydrogen and oxygen to water in sealed car batteries, for purposes ofextending battery life. Cerium oxide is a good ionic conductor and hasbeen used as an electrolyte material of solid oxide fuel cells and gassensors.

Cerium oxide has high dielectric constant and a high refractive indexmaking the material suitable for optical coatings, as discussed, forexample, in Kanakaraju, S., Mohan, S. and Sood, A. K., Thin Solid Films,Vol. 305, Nos. 1-2 (1997), p. 191. Cerium oxide is also of interest as acatalyst in vehicle emissions systems, as discussed in Trovarelli, A.,Boaro, M., Rocchini, E., de Leitenburg, C., and Dolcetti, G., Journal ofAlloys and Compounds, Vol. 323-324 (2001), p. 584, and has also founduse as a solid oxide fuel cell electrolyte material, as reported inSteele, B. C. H. and Heinzel, A., Nature, Vol. 414, No. 6861 (2001), p.345; in gas sensors, as described in Stefanik, T. S. and Tuller, H. L.,Journal of the European Ceramic Society, Vol. 21, Nos. 10-11 (2001), p.1967; in high-T_(c) superconductor structures, as discussed inWalkenhorst, A., Schmitt, M., Adrian, H. and Petersen, K., AppliedPhysics Letters, Vol. 64, No. 14 (1994), p. 1871; andsilicon-on-insulator structures and high storage capacitor devices, asdescribed by Tye, L., El-Masry, N. A., Chikyow, T., Mclarty, P. andBedair, S. M. Applied Physics Letters, Vol. 65, No. 25 (1994), p. 1030.Because of the relative hardness of the material, cerium oxidenanoparticles are also useful as an abrasive for fine polishing ofsurfaces of certain materials, such as quartz and silicon.

Some applications may benefit from using monodispersed cerium oxidenanoparticles, due to the possibility of new properties of cerium oxidein the nanodimension. A method and apparatus for the preparation ofmonodispersed cerium oxide nanoparticles has been described inInternational Patent Application No. PCT/US02/14539 (Attorney Docket No.34284PCT), herein incorporated by reference in its entirety. The smallsize of cerium oxide nanoparticles is also advantageous because theyprovide a relatively large surface area, which increases the oxygenstorage capacity of cerium oxide. However, the ability of thenanoparticles to store oxygen decreases at high temperatures, such asthe temperatures encountered in automotive exhaust systems. Thisdecrease is due to sintering of the nanoparticles at high temperature,which causes at least some nanoparticles to join to form largerparticles. As a result of the formation of larger nanoparticles, theoverall surface area available decreases.

Nanoparticles of cerium oxide which contain zirconium show increasedstability to changes in size upon heating or sintering at hightemperatures while retaining all of the beneficial properties and usesof pure cerium oxide nanoparticles discussed above. Such nanoparticlesare not only a more thermally stable catalyst than nanoparticles of purecerium oxide, but also a more effective catalyst than nanoparticles ofpure cerium oxide in three-way catalysis and water-gas-shift. The effectof zirconium has been discussed by Mamontov, E., Egami, T., Brezny, R.,Koranne, M., and Tyagi, S., J. Phys. Chem. B, Vol. 104, No. 47 (2000),p. 11110, who suggested that the smaller ionic radius of Zr⁴⁺ (0.84 Å)relative to Ce⁴⁺ (0.97 Å) may promote the formation of Ce³⁺ ions, whichmay cause the formation of oxygen vacancies. These vacancies enhance thereactivity of the particles of cerium oxide containing zirconium as acatalyst, as an electrolyte for solid oxide fuel cells, and as a gassensor.

Several methods have been described for preparing particles of ceriumoxide containing zirconium. One approach involves sintering of a mixtureof powders of zirconium oxide and cerium oxide above 1400° C. has beendescribed in Fornasiero, P., Monte, R., Di, G., Rao, R., Kaspar, J.,Meriani, S., Trovarelli, A. and Graziani, M., Journal of Catalysis, Vol.151, No. 1 (1995), pp. 168-177, and in Yashima, M., Takashina, H.,Kakihana, M. and Yoshimura, M., Journal of the American Ceramic Society,Vol. 77, No. 7 (1994), pp. 1869-74. Both the Fornasiero et al. and theYashima et al. methods require a very high sintering temperature andproduce particles with a large particle size and a very large particlesize distribution. Another method involves heating an aqueous mixture of(NH₄)₂Ce(NO₃)₆ and ZrOCl₂.8H₂O having a total molar concentration ofzirconium ions and cerium ions of 0.005 M at 100° C. for 168 hours,followed by high temperature sintering of the precipitate, as describedin Hirano, M., Miwa, T., and Inagaki, M., Journal of Solid StateChemistry, Vol. 158, No. 1 (2001), pp. 112-17. This method involves avery long reaction time and gives a low yield of the nanoparticles. Afurther approach involves mixing urea, (NH₄)₂Ce(NO₃)₆ and ZrOCl₂.8H₂O at100° C. to obtain a gel, boiling the gel for 8 h at 100° C., aging for aperiod of several days, and sintering the resulting mixture at 650° C.,as described in Kundacovic, Lj. and Flytzani-Stephanopoulos, M., Journalof Catalysis, Vol. 179, No. 1 (1998), p. 203. This approach requires along period of time for processing the gel and gives low particleyields. All methods described above require temperatures of at least100° C.

Accordingly, a need exists in the art for an efficient method forpreparing significant quantities of nanoparticles comprising ceriumoxide and zirconium with a relatively narrow size distribution.

SUMMARY OF THE INVENTION

The above-described need in the art is substantially satisfied by themethod of this invention for preparing nanoparticles of cerium oxide andzirconium with a relatively narrow size distribution. In one exemplaryembodiment, the method comprises providing a first aqueous solutioncomprising zirconium oxychloride and providing a second aqueous solutioncomprising a first component which is either cerium nitrate orhexamethylenetetramine. The second aqueous solution is added to thefirst aqueous solution to form a first mixture. A third aqueous solutioncomprising a second component which is either cerium nitrate orhexamethylenetetramine, and which is different from the first component,is added to the first mixture to form a second mixture. The secondmixture is maintained at a temperature no higher than about 320° K. toform nanoparticles. The nanoparticles are then separated from the secondmixture and sintered in air at a temperature ranging between about 500°to about 1100° C.

In another exemplary embodiment, the method comprises providing a firstaqueous solution comprising a first component which is either ceriumnitrate and hexamethylenetetramine and providing a second aqueoussolution comprising a second component which is either cerium nitrateand hexamethylenetetramine and which is different from the firstcomponent. The second aqueous solution is added to the first aqueoussolution to form a first mixture. The first mixture is maintained at atemperature no higher than about 320° K. for about 1 to about 5 hours. Athird aqueous solution comprising zirconium oxychloride is then added tothe first mixture to form a second mixture. The second mixture ismaintained at a temperature no higher than about 320° K. to formnanoparticles. The nanoparticles are then separated from the secondmixture and sintered in air at a temperature ranging between about 500°to about 1100° C.

The method of the invention gives nanoparticles comprising cerium oxideand zirconium having a relatively narrow size distribution. Thenanoparticles require greater thermal energy for particle growth thanpure cerium oxide nanoparticles, and are therefore more thermally stablethan pure cerium oxide nanoparticles while maintaining similar favorableproperties and uses.

The method of the invention has the advantage of being usable to preparenanoparticles comprising cerium oxide and zirconium in a quantity whichis limited only by the size of the mixing vessel. Batches of suchnanoparticles up to about 70 gm have been prepared with the method ofthe invention. This is a very large amount when compared to the scale ofnanoparticle synthesis of the prior art. By providing for a fast initialmixing rate and controlling the reaction time, it is also possible toprepare nanoparticles comprising cerium oxide and zirconium within adesired size,distribution. The method also has the advantage ofproviding crystalline nanoparticles.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 shows plots of X-ray diffraction data for samples of thenanoparticles obtained in accordance with the procedure described inExamples 1-4 herein.

FIG. 2A shows a comparison of the X-ray diffraction peaks for thenanoparticles obtained in accordance with the procedures described inExamples 5-8 herein and the X-ray diffraction peaks of nanoparticles ofpure cerium oxide.

FIG. 2B shows a magnification of the X-ray diffraction peaks of FIG. 2Ain the region ranging from 2θ=45° to 2θ=65°.

FIG. 2C shows a comparison of the X-ray diffraction peaks for thenanoparticles obtained in accordance with the procedures described inExamples 5-8 herein and the X-ray diffraction peaks of pure tetragonalzirconium oxide.

FIG. 2D shows a comparison of the X-ray diffraction peaks for thenanoparticles obtained in accordance with the procedures described inExamples 5-8 herein and the diffraction peaks of particles containingcerium oxide and 25% by mole of zirconium.

FIG. 3A shows a comparison of the X-ray diffraction peaks for thenanoparticles obtained in accordance with the procedure described inExample 9 herein and the nanoparticles obtained in accordance with theprocedure described in Example 11 herein.

FIG. 3B shows a magnification of the X-ray diffraction peaks of FIG. 3Ain the region ranging from 2θ=85° to 2θ=100°.

FIG. 4A shows a comparison of the X-ray diffraction peaks fornanoparticles of an unsintered zirconium-containing precursor tozirconium oxide, nanoparticles of zirconium oxide, and the nanoparticlesobtained in accordance with the procedure described in Example 14herein.

FIG. 4B shows a comparison of the X-ray diffraction peaks for theunsintered nanoparticles obtained in accordance with the proceduredescribed in Example 16 herein and the sintered nanoparticles obtainedin accordance with the procedure described in Example 17 herein.

FIG. 5A shows a tunneling electron microscopy (TEM) image fornanoparticles comprising 64 mole-% cerium oxide and 36 mole-% zirconiumsintered at 600° C.

FIG. 5B shows a tunneling electron microscopy (TEM) image fornanoparticles comprising 64 mole-% cerium oxide and 36 mole-% zirconiumsintered at 900° C.

FIG. 5C shows a tunneling electron microscopy (TEM) image fornanoparticles comprising 64 mole-% cerium oxide and 36 mole-% zirconiumsintered at 1100° C.

FIG. 6 shows a plot of the actual molar percentage of zirconium in thenanoparticles containing cerium oxide and zirconium versus the molarpercentage of zirconium expected on the basis of the initial relativeamounts of the reactants zirconium oxychloride and cerium nitrate.

DETAILED DESCRIPTION OF THE INVENTION

According to an exemplary embodiment of the present invention,nanoparticles containing cerium oxide and zirconium are prepared byplacing in a container a first aqueous solution of zirconiumoxychloride, adding to the first aqueous solution a second aqueoussolution of either cerium nitrate (Ce(NO₃)₃.6H₂O) orhexamethylenetetramine to form a first mixture, and adding to the firstmixture a third aqueous solution of the other of cerium nitrate orhexamethylenetetramine to form a second mixture. The second mixture ismaintained at a temperature no higher than about 320° K. temperature andpreferably at about 300° K. to form nanoparticles containing ceriumoxide and zirconium. The nanoparticles are then separated from thesecond mixture preferably by centrifugation, and the separatednanoparticles are sintered in air at a temperature ranging between about500° C. to about 1100° C. The concentration of the cerium nitrateaqueous solution is in the range of about 0.005 M to 0.1 M, and ispreferably 0.04 M. The concentration of the hexamethylenetetramineaqueous solution is in the range of about 0.01 M to about 1.5 M, and ispreferably in the range of about 0.5 M to about 1.5 M. The concentrationof the zirconium oxychloride aqueous solution is in the range of about0.005 M to about 0.1 M, and is preferably about 0.01 M.

The mixture is continuously stirred with a mechanical stirrer built inthe container, while the temperature of the mixture is maintained atabout 320° K. The mechanical stirrer has a vertical rotating memberpositioned about along the vertical axis of the cylindrical container.The vertical rotating member has a plurality of stirring componentsextending horizontally therefrom. Stirring is performed for a periodbetween about 2 and about 24 hours, preferably between about 5 and about20 hours. The stirrer mixes the mixture at a rotational speed of about50 to about 300 rpm for the duration of the reaction, which depends onthe desired particle size as discussed above. In addition to controllingthe reaction time, the particle size can be monitored by measuring thelight absorption Spectrum of the mixture at different reaction times.

In another exemplary embodiment of the invention, initial thoroughmixing of the reactants may be achieved as follows. The first solutionis first placed in the container. The second and third solutions at arelatively rapid rate are then pumped into the container containing thefirst solution through a plurality of inlets which are distributedthroughout the inner wall of the container, such that turbulence iscreated in the mixture in the container to ensure initial thoroughmixing of the three solutions. In this embodiment of the invention, thecontainer comprises one or more detachable plastic liners which adhereto the walls of the container. The liners are made from a chemicallyinert material, such as TEFLON®, plastic or polyethylene.Advantageously, the second solution is pumped at high pressure to eusureinitial rapid and thorough mixing and nucleation of the nanoparticles atapproximately the same time. The nanoparticles grow at a uniform rateand thereby achieve monodispersity. Upon completion of the reaction, thereaction mixture may be centrifuged as described herein, whereby theparticles are deposited on the detachable liners covering the inner wallof the mixing vessel. The nanoparticles may then be obtained bydetaching the liners from the wall of the mixing vessel.

The size of the nanoparticles obtained from the reaction mixtureincreases with increasing length of the reaction time. Largernanoparticles are obtained when the mixing is carried out for about 12to about 24 hours.

The nanoparticles may be separated from the reaction mixture bycentrifugation. In an advantageous embodiment of the invention, thecontainer is positioned inside a centrifuge. The suspension that resultsfrom the formation of the nanoparticles in the aqueous medium can becentrifuged at about 9,000 rpm or higher to separate the particles andthe supernatant when the particles have reached a desired size. The timerequired for separation by centrifugation depends on the particle size.In general, the time required is readily calculated from standardcentrifugation equations for separating particles from a liquidsuspension, as is known to persons of ordinary skill in the art. Theprocess of separation is effective due to the substantial difference inthe densities of the supernatant (ρ≅1 gm/cm³) and of the nanoparticlescontaining cerium oxide (ρ≅7.2 gm/cm³) and zirconium oxide (ρ≅5.68gm/cm³).

The sintering step is advantageously carried out for about 0.5 hours toabout 5 hours, preferably I hour, at temperatures ranging between 500°C. and 1100° C. The sintering temperature is ramped up at a rate of 100°C./hour for about 4.8 hours to about 10.8 hours starting with an initialtemperature of about 20° C. The sintering temperature is preferablymaintained at its maximum value for about 30 minutes, after which thesintering temperature is ramped down at a rate of −100° C./hour forbetween about 4.8 hours and about 10.8 hours until the nanoparticlesreturn to the initial temperature of about 20° C.

The nanoparticles obtained after centrifugation are at least in partcrystalline, and the particle sizes may be measured by X-raydiffraction. All X-ray diffraction experiments may be performed using adiffractometer of model Scintag X2 with Cu Kα irradiation under the sameconditions, including the same scan rate (0.025 degree/step, 5 s/step).The lattice parameter a is determined from fitting the x-ray diffractionpeak position. A scanning-range of 20 degrees to 135 degrees was used.The mean particle diameter χ₀ is determined from the peak width usingthe Scherrer formula, χ₀=0.94λ/B cosθ_(B), where λ is the wavelength ofthe Cu Kα₁ line, θ_(B) is the angle between the incident beam and thereflecting lattice planes, and B is the width (in radians) of thediffraction peak. The size dispersion is approximately gaussian with afall width at the half-maximum value which is about 34% of the meandiameter or less, depending on the sintering temperature. From the x-raydiffraction data, peak positions were found using the Kolaire program asdescribed in Cheary, R. W. & Coelho, A. A., Programs XFIT and FOURYA(1996), deposited in CCP14 Powder Diffraction Library, Engineering andPhysical Sciences Research Council, Daresbury Laboratory, Warrington,England. (http://www.ccp14.ac.uk/tutorial/xfit-95/xfit.htm). Thesepositions were used in the Celref program for the determination of thelattice parameter as described in Laugier, J. and Bochu, B., Celref forWindows unit cell refinement program, ENSP/Laboratoire des Matériaux etdu Génie Physique, France, and as further described athttp://www.inpg.fr/LMPG.

Average particle size was also determined. Peak positions andintensities were compared with the plots in the standard Joint Committeeon Powder Diffraction Standards—Powder Diffraction File (hereinafter“JCPDS-PDF”) and used for compound identification.

Crystals of the nanoparticles obtained after centrifugation step andbefore the sintering step do not contain zirconium in the form ofzirconium oxide. FIG. 1 shows the X-ray diffraction plot for Samples 1-4of nanoparticles obtained in accordance with the procedure described inExamples 1-4 herein, which does not include a sintering step. The molarpercentage of zirconium relative to the moles of cerium oxide andzirconium was equal to 10%, 20%, 30%, and 40% in the samples obtained inExamples 1, 2, 3, and 4, respectively. The resulting X-ray diffractionplots show that there was no change between the diffraction peaks ofpure cerium oxide, obtained from the JCPDS-PDF database and thediffraction peaks of the nanoparticles of Examples 1-4.

If the nanoparticles obtained after centrifugation are subjected tosintering, zirconium is incorporated into the cerium oxidenanoparticles. As described in Examples 5-8 herein, the nanoparticlesobtained in Examples 1-4 were subjected to a sintering step at 550° C.to give sintered nanoparticle Samples 5-8, respectively, which contain10%, 20%, 30%, and 40% zirconium as a molar percentage of the moles ofcerium oxide and zirconium. The X-ray diffraction plots of Samples 5-8are shown in FIGS. 2A-2D together with the vertical lines representingthe positions of X-ray diffraction peaks of pure cerium oxide,tetragonal zirconium oxide, and micron-sized particles containing ceriumoxide and 25% zirconium, respectively. The diffraction peaks for each ofthe sintered nanoparticle samples are shifted relative to the peaks ofpure cerium oxide (FIG. 2A), as can be clearly seen from themagnification of the X-ray diffraction peaks in the region ranging from2θ=45° to 2θ=65° shown in FIG. 2B. The diffraction peaks for thesintered nanoparticle samples resemble the JCPDS-PDF peaks fortetragonal zirconium oxide (FIG. 2C) and the JCPDS-PDF peaks formicron-sized particles containing cerium oxide and 25% zirconium oxide(FIG. 2D).

Without wishing to be bound by any mechanism or theory, it is believedthat the reaction between cerium nitrate and hexamethylenetetraminegives cerium oxide, and that the reaction between zirconium oxychlorideand hexamethylenetetramine gives an unsintered zirconium-containingprecursor to zirconium oxide. The X-ray diffraction plot in FIG. 4A forthe nanoparticles obtained from Example 13 herein, in which onlyzirconium oxychloride and hexamethylenetetramine were reacted, issimilar to the JCPDS-PDF plot for hexamethylenetetramine, indicatingthat the unsintered zirconium-containing precursor to zirconium oxide isan amorphous material. Upon sintering, the zirconium oxide is formed andthe tetravalent zirconium ions of zirconium oxide diffuse into thecerium oxide lattice and substitute the tetravalent cerium ions. As isfurther discussed below, as the amount of zirconium incorporated in thelattice increases, the nanoparticles containing cerium oxide andzirconium begin to have both tetragonal and fluorite structures, ratherthan only the fluorite structure which is more stable for pure ceriumoxide crystais.

Similar results were found for the nanoparticles obtained in accordancewith the procedure described in Example 9 herein, which does not includea sintering step, in which cerium oxide nanoparticles were initiallyformed by mixing cerium nitrate and hexamethylenetetramine in theabsence of zirconium oxychloride. Zirconium oxychloride was then addedto the mixture containing the cerium oxide nanoparticles and unreactedhexamethylenetetramine and cerium nitrate. A comparison of the X-raydiffraction plots showed no significant difference between pure ceriumoxide, the plot of which is shown in FIG. 2A, and the nanoparticles ofSample 9, obtained in Example 9, the plot of which is shown in FIG. 3A.The Sample 9 nanoparticles were then sintered at 900° C. as described inExample 11 herein to give Sample 11. The resulting X-ray diffractionplot (FIG. 3B) shows shifts in the Sample 11 peaks relative to the peaksof pure cerium oxide shown in FIG. 2A and to the peaks of thenanoparticle Sample 9, indicating that Sample 11 contained zirconium.The peak shifts in Sample 11 can be clearly seen from the magnificationof the X-ray diffraction peaks in the region ranging from 2θ=85° to2θ=100° shown in FIG. 3B. The peak shift in Sample 11 was greater thanin the nanoparticle Sample 6, the diffraction peaks of which are shownin FIGS. 2A-D. This difference in shift is believed to be due to thelower sintering temperature (550° C.) used in Example 6 relative to thesintering temperature (900° C.) used in Example 11.

Without wishing to be bound by any theory or mechanism, it is believedthat the zirconium oxychloride added to the mixture reacts withhexamethylenetetramine to give a zirconium-containing precursor tozirconium oxide. The formation of this precursor to zirconium oxide isbelieved to disrupt the formation of the cerium oxide lattice, therebypreventing further growth of the cerium oxide nanoparticles. Theparticles obtained before sintering include a mixture of thezirconium-containing precursor to zirconium oxide and cerium oxide, andmay further contain a mixed oxide of formula Zr_(x)Ce_(1-x)O₂.yH₂O. Thiscompound is formed after the mixing step in our method, where x isbetween 0 and 1. Upon sintering, the zirconium ions diffuse into thecerium oxide lattice as previously discussed.

Similar results were observed for the nanoparticles of Samples 16 and 17obtained in accordance with the procedure described in Examples 16 and17 herein, respectively. Zirconium oxychloride andhexamethylenetetramine were mixed for an amount of time-sufficient toallow formation of a precipitate. Without wishing to be bound by anytheory or mechanism, the precipitate is believed to be azirconium-containing precursor to zirconium oxide. A cerium nitratesolution was then added to the mixture containing the precipitate toform Sample 16. The X-ray diffraction plot of Sample 16 (FIG. 4B) wasfound to be very similar to the X-ray diffraction plot for pure ceriumoxide. Sample 16 was then sintered to form Sample 17. Comparison of theX-ray diffraction plots of Sample 16 and of Sample 17 (FIG. 4B) shows aclear peak shift to larger 2θ values for Sample 17, indicating formationof nanoparticles containing cerium oxide and zirconium in Sample 17.

The nanoparticles containing cerium oxide and zirconium obtained aftersintering are monodispersed, wherein the term as used herein is intendedto mean particles; in which the full width at half maximum (FWHM) of thesize distribution peak for a batch of 100 or more particles is less than+/−35% of the median size. FIGS. 5A-5C show tunneling electronmicroscopy (TEM) images for nanoparticles comprising 64% cerium oxideand 36% zirconium sintered at 600° C., 900° C. and 1100° C.,respectively. As shown in Table 1, the diameter of the nanoparticlesobtained from TEM is about 6.5 nm for nanoparticles sintered at about600° C., about 9 nm for nanoparticles sintered at about 900° C., andabout 11 nm for nanoparticles sintered at about 1100° C. Themonodispersity for the nanoparticles shown in Table 1 varies from about29% for nanoparticles sintered at about 900° C. to about 34% fornanoparticles sintered at about 1100° C. TABLE 1 Particle size and sizedistribution for nanoparticles comprising 64% cerium oxide and 36%zirconium oxide. Sintering Size Distribution Temperature Particle SizeSize Distribution (% of median (° C.) (nm) (nm) particle diameter) 6006.45 ±2.01 ±31 900 9.18 ±2.66 ±29 1100 11.13 ±3.83 ±34

The amount of zirconium which is incorporated into the nanoparticles islarger than the expected amount based on the molar percentages ofzirconium oxychloride and cerium nitrate reactants. Without wishing tobe bound by any theory or mechanism, the incorporation of a larger thanexpected amount of zirconium in the cerium oxide nanoparticles is due tothe lower pH of a zirconium oxychloride solution relative to a solutionof cerium nitrate in an equal concentration. The lower pH corresponds toa greater reactivity of zirconium oxychloride withhexamethylenetetramine. As shown in Table 2, for an initial ratio ofzirconium oxychloride and cerium nitrate corresponding to an expectedpercentage of 20% of zirconium in the nanoparticles, the molarpercentage of zirconium in the nanoparticles containing cerium oxide andzirconium was found to be 36% at sintering temperatures of 550° C., 900°C., and 1100° C. The presence of zirconium in the nanoparticles alsoleads to a reduction in the lattice parameter compared to pure ceriumoxide, as shown in Table 2. This reduction is believed to be due to thesmaller ionic radius of Zr⁴⁺ relative to the ionic radius of Ce⁴⁺. Therelationship between expected and actual amounts of zirconium isillustrated in FIG. 6. The slope of the figure is about 1.79, whichmeans that for an expected value of 30% by mole of zirconium, the actualvalue is about 54%, or more than half of the total amount of oxides inthe nanoparticle. This result may explain why the nanoparticles obtainedusing amounts of zirconium oxychloride corresponding to expected valuesof 30% and 40% of zirconium have both tetragonal and fluoritestructures, rather than only the fluorite structure which is more stablefor pure cerium oxide crystals. TABLE 2 Comparison of Expected andActual Molar Percentage Composition of Nanoparticles Actual CompositionExpected (by Inductively Lattice Particle Size composition CoupledPlasma) Parameter a(Å) d(nm) Pure CeO₂ Pure CeO₂ 5.4330 6.1 Pure CeO₂Pure CeO₂ 5.411 5000 20% ZrO₂-CeO₂ 36% ZrO₂-CeO₂ 5.3921 5 Sintered-550°20% ZrO₂-CeO₂ 36% ZrO₂-CeO₂ 5.3726 6 Sintered-900° 36% ZrO₂-CeO₂ 36%ZrO₂-CeO₂ 5.3174 Micron sized Predict from JCPDF 25% ZrO₂-CeO₂ 25%ZrO₂-CeO₂ 5.349 Micron sized from JCPDF

The invention is further described in the following examples, which areintended to be illustrative and not limiting of the scope of theinvention.

EXAMPLES Examples 1-4 Preparation of Unsintered Nanoparticle Samples 1-4

For the preparation of Sample 1, the following aqueous solutions wereprepared: 375 ml of 0.072 M cerium nitrate, 375 ml of 0.008 M zirconiumoxychloride, and 750 ml of 0.5 M hexamethylenetetramine (HMT). Each ofthese three solutions was stirred for 20 minutes. The HMT solution andthe cerium nitrate solution were then added in rapid succession to thezirconium oxychloride solution. The resulting mixture had molarpercentages of 10% zirconium and 90% cerium. This mixture was stirredovernight for approximately 20 hours. The resulting mixture after suchstirring was centrifuged at 9000 rpm for 30 minutes to recover theprecipitate. The precipitate was then dried in an oven at 40° C. For thepreparation of Samples 2,3 and 4, the same procedure was used with themodification that the molarity of the starting solutions was changed sothat the amount of zirconium as a molar percentage of the cerium amountin the final solution was 20%, 30% and 40%, respectively, correspondingto the expected percentage of zirconium in the nanoparticles. Allsamples were light brown in color.

Examples 5-8 Preparation of Sintered Nanoparticle Samples 5, 6, 7, 8

These samples were prepared by sintering Samples 1, 2, 3 and 4,respectively, according to the following procedure. The temperature wasramped up at a rate of 100° C. per hour for 5.3 hours from 20° C. tobring the temperature up to 550° C. The temperature was held at 550° C.for 1 hour. The temperature was then ramped down at a rate of −100° C.per hour for 5.30 hours to reach a final temperature of 20° C. Allsamples were yellow in color. The particle diameter of Sample 6 wasdetermined to be ˜5 nm, determined by X-ray diffraction.

Examples 9-10 Preparation of Unsintered Nanoparticle Samples 9-10

For the preparation of Sample 9, the following starting solutions wereprepared: 0.0356 M cerium nitrate (1.835 g in 150 mL water), 0.0133 Mzirconium oxychloride (0.430 g in 100 mL water), 0.5 M HNMT (7.011 g in100 mL water). Each of these three solutions was stirred for 20 minutes.The HMT solution and the cerium nitrate solution were combined, and theresulting mixture was stirred for 2 hours. The zirconium oxychloridesolution was then added and the resulting second mixture had molarpercentages of 20% zirconium and 80% cerium. This second mixture wasstirred overnight for approximately 20 hours. The second mixture aftersuch stirring was centrifuged at 9000 rpm for 30 minutes to recover theprecipitate. The precipitate was then dried in an oven at 40° C. For thepreparation of Sample 10 the above procedure was used with themodification that the molarity of the starting solutions were changed sothat the zirconium molar percentage in the final solution was 30%.Samples were light brown in color.

Example 11 Preparation of Sintered Nanoparticle Sample 11

Sample 11 was prepared by sintering Sample 9 according to the followingprocedure. The temperature was ramped up at a rate of 100° C. per hourfor 8.8 hours bringing the temperature up to 900° C. The temperature washeld at 900° C. for 1 hour. The temperature was then ramped down at arate of −100° C. per hour for 8.8 hours to reach a final temperature of20° C. The sample was yellow in color. The particle diameter of Sample11 was determined to be ˜6 nm.

Example 12 Preparation of Unsintered Nanoparticle Sample 12

For the preparation of Sample 12, the following starting solutions wereprepared: 0.007 M Cerium nitrate (0.120 g in 100 mL water), 0.003 Mzirconium oxychloride (0.0967 g in 100 mL water), 0.05M HMT (0.7011 g in100 mL water). Each of these three solutions was stirred for 20 minutes.After this time, the HMT solution and Cerium nitrate solution were addedto the zirconium oxychloride solution. The resulting mixture had molarpercentages of 30% zirconium and 70% cerium. This solution was stirredovernight for approximately 20 hours. The resulting mixture after suchstirring was centrifuged at 9000 rpm for 30 minutes to recover theprecipitate. The precipitate was then dried in an oven at 40° C. Sample12 was very light yellow-brown in color.

Example 13 Preparation of Unsintered Nanoparticle Sample 13

For the preparation of Sample 13, the following solutions were prepared:0.03 M zirconium oxychloride (1.289 g in 100 mL of water) and 0.5M HMT(3.506 g in 100 mL of water). Both solutions were stirred for 20minutes. After stirring, the HMT solution was added to the zirconiumoxychloride solution. The resulting mixture was stirred for 1 hour. Awhite precipitate slowly formed as the mixture was being stirred. Theresulting mixture after such stirring was centrifuged at 9000 rpm for 30minutes. The precipitate, Sample 13, was left to dry at roomtemperature.

Example 14 Preparation of Sintered Nanoparticle Sample 14

Sample 13, obtained in Example 13, was sintered to form Sample 14according to the sintering procedure described in Examples 5-8. Sample14 is tetragonal zirconium oxide, as shown by comparison of X-raydiffraction peaks of the sample with the JPCDS data for tetragonalzirconium oxide (FIG. 4A).

Example 15 Preparation of Sintered Nanoparticle Sample 15

For the preparation of Sample 15, the following starting solutions wereprepared: 0.0534 M cerium nitrate (9.175 g in 500 mL water), 0.0133 Mzirconium oxychloride (2.1485 g in 500 mL water), 0.5 M HMT (35.055 g in500 mL water). Each of these three solutions was stirred for 20 minutes.After this time, the HMT solution and cerium nitrate were added to thezirconium oxychloride solution. The resulting mixture had molarpercentages of 20% zirconium and 80% cerium. This mixture was stirredfor 20 hours. The resulting mixture after stirring was centrifuged at10000 rpm for 30 minutes to recover the precipitate. The precipitate wasthen dried in an oven at 40° C. The precipitate was sintered accordingto the procedure of Examples 5-8.

Example 16 Preparation of Unsintered Nanoparticle Sample 16

For the preparation of Sample 16, the following solutions were prepared:150 mL of 0.0356 M cerium nitrate, 100 mL of 0.0133 M zirconiumoxychloride, and 100 mL 0.5 M HMT. Each of these three solutions wasstirred for 30 minutes. The HMT solution and the zirconium oxychloridesolution were combined to give a mixture which was stirred for 2 hours.The cerium nitrate solution was then added to the mixture to give a newmixture having 20% molar zirconium and 80% molar cerium. The new mixturewas stirred overnight for 20 hours. The new mixture was then centrifugedat 9000 rpm for 30 minutes to form a precipitate and a liquid phase. Theprecipitate was separated from the liquid phase and dried in an oven at40° C.

Example 17 Preparation of Sintered Nanoparticle Sample 17

Sample 16, obtained in Example 16, was sintered to form Sample 17according to the following procedure. The temperature was ramped up at arate of 100° C. per hour for 5.8 hours bringing the temperature up to600° C. The temperature was held at 600° C. for 1 hour. The temperaturewas then ramped down at a rate of −100° C. per hour for 5.8 hours toreach a final temperature of 20° C. The resulting Sample 17 was yellowin color.

It should be understood that various changes and modifications to theexemplary embodiments described herein will be apparent to those skilledin the art without departing from the spirit and scope of thisinvention, the scope being defined by the appended claims.

1. A method for preparing nanoparticles comprising cerium oxide andzirconium, wherein the method comprises the steps of: (a) providing afirst aqueous solution comprising zirconium oxychloride; (b) mixing thefirst aqueous solution with a second aqueous solution comprising a firstcomponent selected from one of cerium nitrate and hexamethylenetetramineto form a first mixture; (c) mixing the first mixture with a thirdaqueous solution comprising a second component selected from ceriumnitrate and hexamethylenetetramine to form a second mixture, wherein thesecond component is different from the first component; (d) maintainingthe second mixture at a temperature no higher than about 320° K to formnanoparticles therein; (e) separating the nanoparticles formed in step(d) from the second mixture; and (f) sintering the nanoparticlesseparated in step (e) in air at a temperature ranging between about 500°C. to about 1100° C.
 2. The method of claim 1, wherein the first aqueoussolution has a concentration of zirconium oxychloride ranging from about0.005 M to about 0.1 M.
 3. The method of claim 1, wherein the secondaqueous solution comprises cerium nitrate in a concentration rangingfrom about 0.005 M to about 0.1 M.
 4. The method of claim 1, wherein thethird aqueous solution comprises cerium nitrate in a concentrationranging from about 0.005 M to about 0.1 M.
 5. The method of claim 1,wherein the second aqueous solution comprises hexamethylenetetramine ina concentration ranging from about 0.01 M to about 1.5 M.
 6. The methodof claim 1, wherein the third aqueous solution compriseshexamethylenetetramine in a concentration ranging from about 0.01 M toabout 1.5 M.
 7. The method of claim 5, wherein the second aqueoussolution comprises hexamethylenetetramine in a concentration ofhexamethylenetetramine ranging from about 0.5 M to about 1.5 M.
 8. Themethod of claim 6, wherein the third aqueous solution compriseshexamethylenetetramine in a concentration of hexamethylenetetramineranging from about 0.5 M to about 1.5 M.
 9. The method of claim 1,wherein step (d) comprises stirring the second mixture while it is beingmaintained at a temperature no higher than about 320° K to formnanoparticles therein.
 10. The method of claim 1, wherein the first andsecond mixtures are formed in a container having a mechanical stirrer,and the first mixture and the third aqueous solution are mixed with themechanical stirrer to form the second mixture.
 11. The method of claim1, wherein step (e) comprises maintaining the second mixture at atemperature no higher than about 320° K for a time period between about2 hours and about 24 hours.
 12. The method of claim 11, wherein the timeperiod is between about 5 hours and about 24 hours.
 13. The method ofclaim 12, wherein the time period is between about 12 hours and about 24hours.
 14. The method of claim 1, wherein step (e) comprisescentrifuging the second mixture to separate the nanoparticles from thesecond mixture.
 15. The method of claim 1, wherein the second mixture isformed in a container, and the method comprises positioning thecontainer inside a centrifuge and centrifuging the second mixture afterformation of the nanoparticles therein for separating the nanoparticlesfrom the second mixture.
 16. The method of claim 1, wherein thenanoparticles separated in step (e) are at least in part, crystalline.17. The method of claim 1, wherein after step (f) is performed, thenanoparticles are at least in part crystalline.
 18. The method of claim1, wherein the sintering of the nanoparticles in step (f) takes place ata temperature of about 550° C.
 19. The method of claim 1, wherein thesintering of the nanoparticles in step (f) takes place at a temperatureof about 900° C.
 20. The method of claim 1, wherein the molar percentageof zirconium in the nanoparticles comprising cerium oxide and zirconiumis in the range of about 20% to about 75%.
 21. The method of claim 1,wherein the first aqueous solution is provided in a container, and thesecond and third aqueous solutions are mixed with the first solution andthe first mixture, respectively, by pumping the second and third aqueoussolutions into the container through a plurality of inlets which aredistributed throughout the container.
 22. A method for preparingnanoparticles comprising cerium oxide and zirconium, wherein the methodcomprises the steps of: (a) providing a first aqueous solutioncomprising a first component selected from one of cerium nitrate andhexamethylenetetramine; (b) mixing the first aqueous solution with asecond aqueous solution comprising a second component selected from oneof cerium nitrate and hexamethylenetetramine to form a first mixture,wherein the second component is different from the first component; (c)maintaining the first mixture at a temperature no higher than about 320°K for about 1 to about 5 hours; (d) after step (c) mixing the firstmixture with a third aqueous solution comprising zirconium oxychlorideto form a second mixture; (e) maintaining the second mixture at atemperature no higher than about 320° K to form nanoparticles therein;(f) separating the nanoparticles formed in step (e) from the secondmixture; and (g) sintering the nanoparticles separated in step (f) inair at a temperature in the range of about 500° C. to about 1100° C. 23.A method for preparing nanoparticles comprising cerium oxide andzirconium, wherein the method comprises the steps of: (a) providing afirst aqueous solution comprising zirconium oxychloride; (b) mixing thefirst aqueous solution with a second aqueous solution comprisinghexamethylenetetramine to form a first mixture, wherein the first andsecond aqueous solution are mixed for an amount of time sufficient toallow formation of a precipitate in the first mixture; (c) mixing thefirst mixture with a third aqueous solution comprising a cerium nitrateto form a second mixture; (d) maintaining the second mixture at atemperature no higher than about 320° K to form nanoparticles therein;(e) separating the nanoparticles formed in step (d) from the secondmixture; and (f) sintering the nanoparticles separated in step (e) inair at a temperature ranging between about 500° C. to about 1100° C. 24.The method of claim 23, wherein the first and second aqueous solutionare mixed in step (b) for between about 1 hour to about 2 hours.